Nitride semiconductor light emitting device

ABSTRACT

A nitride semiconductor light emitting device includes n-type and p-type nitride semiconductor layers, an active layer disposed between the n-type and p-type nitride semiconductor layers and having a structure in which a plurality of quantum barrier layers and one or more quantum well layers are alternately stacked, and an electron blocking layer disposed between the active layer and the p-type nitride semiconductor layer. The electron blocking layer has greater bandgap energy than a quantum barrier layer adjacent to the electron blocking layer among the plurality of quantum barrier layers, and has a net polarization equal to or smaller than that of the quantum barrier layer adjacent thereto. The nitride semiconductor light emitting device can achieve high efficiency in every current region by minimizing a net polarization mismatch between a quantum barrier layer and an electron blocking layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Application No. 60/956,723 filed on Aug. 20, 2007, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor light emitting device, and more particularly, to a nitride semiconductor light emitting device capable of minimizing a decrease in light emission efficiency at high currents.

2. Description of the Related Art

A light emitting diode (LED) is a semiconductor device that can emit light of various colors due to electron-hole recombination occurring at a p-n junction when a current is applied thereto. Compared to conventional lighting sources such as incandescent lighting bulbs and fluorescent lamps, LED has many advantages such as a long lifespan, low power, excellent initial-operation characteristics, and high tolerance to repetitive power on/off. Hence the demand for LED is continuously increasing. Particularly, group III nitride semiconductors that can emit light in the blue/short wavelength region have recently drawn much attention.

However, in a light emitting device using the group III nitride semiconductor, electrons that have a higher mobility than holes flow to a p-type semiconductor layer without combining with the holes. This, as shown in FIG. 1, causes an electron leakage current to increase as the magnitude of currents being injected increases. The increase in the electron leakage current is called electron overflow. FIG. 1 is a graph showing a change in the electron leakage current according to injection currents in a related art nitride semiconductor light emitting device.

The electron leakage current decreases quantum efficiency, and is becoming a more crucial limitation because LEDs are increasingly being used at high currents as in lighting devices. However, no methods have been proposed to completely overcome this limitation. Therefore, there is a need for a high-efficiency nitride semiconductor light emitting device that has high quantum efficiency in every current region, especially at high currents, and thus can be used for a lighting device or the like.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a nitride semiconductor light emitting device which can achieve high efficiency by minimizing a net polarization mismatch between a quantum barrier layer and an electron blocking layer.

According to an aspect of the present invention, there is provided a nitride semiconductor light emitting device including: n-type and p-type nitride semiconductor layers; an active layer disposed between the n-type and p-type nitride semiconductor layers and having a structure in which a plurality of quantum barrier layers and one or more quantum well layers are alternately stacked; and an electron blocking layer disposed between the active layer and the p-type nitride semiconductor layer. The electron blocking layer has greater bandgap energy than a quantum barrier layer adjacent to the electron blocking layer among the plurality of quantum barrier layers, and has a net polarization equal to or smaller than that of the quantum barrier layer adjacent thereto.

The electron blocking layer may have a net polarization which is smaller than or equal to that of GaN and is greater than that of Al_(0.25)Ga_(0.75)N. The electron blocking layer may have bandgap energy having the same magnitude as that of Al_(0.25)Ga_(0.75)N.

The electron blocking layer may have a net polarization which is equal to that of the quantum barrier layer adjacent to the electron blocking layer among the plurality of quantum barrier layers.

A net polarization mismatch at an interface between the electron blocking layer and the quantum barrier layer adjacent to the electron blocking layer may be smaller than a net polarization mismatch between GaN and Al_(x)Ga_((1−x))N (0.1≦x≦0.25). The net polarization mismatch at the interface between the electron blocking layer and the quantum barrier layer adjacent to the electron blocking layer may be half the net polarization mismatch between GaN and Al_(x)Ga_((1−x))N (0.1≦x≦0.25).

The nitride semiconductor light emitting device may further include a substrate for growth contacting the n-type nitride semiconductor layer. The n-type nitride semiconductor layer may be formed on a polar surface of the substrate. The n-type nitride semiconductor layer may be formed on a C (0001) plane of a sapphire substrate.

According to another aspect of the present invention, there is provided a nitride semiconductor light emitting device including: n-type and p-type nitride semiconductor layers; an active layer disposed between the n-type and p-type nitride semiconductor layers and having a structure in which a plurality of quantum barrier layers and one or more quantum well layers are alternately stacked; and an electron blocking layer disposed between the active layer and the p-type nitride semiconductor layer, wherein the electron blocking layer has a constant energy level of a conduction band.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph showing a change in the electron leakage current according to an injection current in a related art nitride semiconductor light emitting device;

FIG. 2A is a cross-sectional view of a nitride semiconductor light emitting device according to an exemplary embodiment of the present invention;

FIG. 2B is an enlarged view of an active layer region of FIG. 2A;

FIG. 3 illustrates changes in bandgap energy and net polarizations according to Al(x) and In(y) compositions in an AlInGaN quaternary semiconductor;

FIG. 4 is a graph showing a simulation result of current changes according to a driving voltage change in embodiments 1 and 2 and a comparison example (Al_(0.13)Ga_(0.87)N electron blocking layer) selected as shown in Table 1;

FIG. 5 is a graph of a simulation result of changes in electron leakage current according to a current change in the embodiment 1 (solid line) and the comparison example (dotted line);

FIG. 6 is a graph of a simulation result of changes in internal quantum efficiency according to a current change in the embodiment 1 (solid line) and the comparison example (dotted line); and

FIG. 7 is a cross-sectional view illustrating a modification version of the nitride semiconductor light emitting device of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. In the figures, the dimensions and the shapes of elements are exaggerated for clarity of illustration. Like reference numerals refer to like elements throughout.

FIG. 2A is a cross-sectional view of a nitride semiconductor light emitting device according to an exemplary embodiment of the present invention. FIG. 2B is an enlarged view of an active layer region of FIG. 2A. Referring to FIGS. 2A and 2B, a nitride semiconductor light emitting device 200 according to the current embodiment includes an n-type nitride semiconductor layer 202, an active layer 203, an electron blocking layer 204 and a p-type nitride semiconductor layer 205 sequentially disposed on a substrate 201. N-type and p-type electrodes 206 a and 206 b are disposed at predetermined regions of the n-type and p-type nitride semiconductor layers 202 and 205, respectively.

The substrate 201 is provided for growth of a nitride semiconductor layer, and a sapphire substrate may be used as the substrate 201. The sapphire substrate is formed of a crystal having Hexa-Rhombo R3c symmetry, and has a lattice constant of 13.001 Å along a C-axis and a lattice constant of 4.758 Å along an A-axis. Orientation planes of the sapphire substrate include a C (0001) plane, an A (1120) plane, an R (1102) plane, etc. Particularly, the C plane is mainly used as a substrate for nitride growth because it relatively facilitates the growth of a nitride film and is stable at a high temperature.

The C plane is a polar plane. A nitride semiconductor layer grown from the C plane has a spontaneous polarization because of intrinsic ionicity of a nitride semiconductor and structural asymmetry (lattice constant a≠c). If nitride semiconductors having different lattice constants are successively stacked, a strain occurring at each semiconductor layer causes a piezoelectric polarization. The sum of those two polarizations is called a net polarization. Net polarization mismatch is formed at each interface by the net polarization, thereby bending an energy-level. The energy-level bending in an active layer causes spatial mismatch between wave functions of electrons and holes, lowering the light emission efficiency. A technique for improving the light emission efficiency by reducing an influence of polarizations will be described in detail. As the substrate 201 for growth of the nitride semiconductor, a substrate formed of SiC, Si, GaN, AlN or the like may be used instead of the sapphire substrate.

In the current embodiment, a nitride semiconductor light emitting device having a horizontal structure including the substrate 201 for growth of a nitride semiconductor is described. However, the present invention is not limited thereto and may be applied to a nitride semiconductor light emitting device having a vertical structure in which electrodes face each other in a stacked direction of semiconductor layers with the substrate 201 removed.

The n-type nitride semiconductor layer 202 and the p-type nitride semiconductor layer 205 may be formed of semiconductor materials having a composition formula Al_(x)In_(y)Ga_((1−x−y))N (0≦x≦1, 0≦y≦1 and 0≦x+y≦1) and doped with n-type impurities and p-type impurities, respectively. Representative examples of the semiconductor material include GaN, AlGaN and InGaN. Si, Ge, Se, Te or the like may be used as the n-type impurities, and Mg, Zn, Be or the like may be used as the p-type impurities. With respect to growth of a nitride semiconductor layer, a known process may be used for the n-type and p-type nitride semiconductor layers 202 and 205. For example, the known process may be, e.g., metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or hydrid vapor-phase epitaxy (HVPE).

As shown in FIG. 2B, the active layer 203 has a stack structure in which quantum barrier layers 203 a and quantum well layers 203 b are alternately stacked on top of each other so as to emit light through electron-hole recombination at the quantum well layers 203 b. In this case, the quantum barrier layer 203 a is formed of GaN, and the quantum well layer 203 b may be formed of In_(0.2)Ga_(0.8)N. The electron blocking layer 204 is disposed between the active layer 203 and the p-type nitride semiconductor layer 205, and has higher bandgap energy than the quantum barrier layer 203 a. Accordingly, the electron blocking layer 204 prevents electrons from overflowing to the p-type nitride semiconductor layer 205.

According to the current embodiment, a net polarization mismatch between the electron blocking layer 204 and the adjacent quantum barrier layer 203 a is smaller than that of a conventional quantum barrier layer/electron blocking layer structure. Accordingly, if the net polarization mismatch at an interface is made to be smaller than a related art, for example, a GaN quantum barrier layer/Al_(0.13)Ga_(0.87)N electron blocking layer structure, energy band bending at the electron blocking layer and the adjacent quantum barrier layer decreases, thereby the electron leakage current over the electron blocking layer decreases. Thus, as will be described later, the driving voltage and electron leakage current decrease, and the light emission efficiency can be improved.

Hereinafter, a method for reducing the net polarization mismatch between the quantum barrier layer and the electron blocking layer according to the current embodiment and effects thereof will now be described. FIG. 3 illustrates changes in bandgap energy and net polarizations with respect to Al(x) and In(y) compositions in an AlInGaN quaternary semiconductor. In this case, the compositions of the same bandgap energy are indicated by a dotted line, and the compositions of the same net polarization are indicated by a solid line. The graph of FIG. 3 is obtained from a calculation considering a bowing parameter and a lattice constant of each element after determining the net polarizations and the bandgap energy of AlN, InN and GaN grown on the GaN layer at a temperature of 300 K.

Referring to FIG. 3, as the Al content of the AlInGaN semiconductor layer increases, the bandgap energy increases and the net polarization decreases. Also, as the In content of the AlInGaN semiconductor layer increases, the bandgap energy decreases and the net polarization increases. However, the degree of changes in bandgap energy and net polarization varies according to changes in Al and In contents. Thus, it can be seen that if the Al and In contents are properly controlled, the net polarization can be reduced while the constant bandgap energy is maintained.

In more detail, as shown in FIG. 3, the bandgap energy of a GaN quantum barrier layer is 3.4200 eV, and the net polarization is −0.0339 C/m². In this case, the minus quantity of the net polarization may be understood as indicating that the GaN quantum barrier layer is positively charged toward a GaN layer at a lower side, and is negatively charged at the opposite side. Also, in the case of a related art Al_(0.13)Ga_(0.87)N electron blocking layer, the bandgap energy is 3.658 eV, and the net polarization is −0.0423 C/m². According to the current embodiment, two compositions are determined for formation of electron blocking layers (embodiments 1 and 2) such that the bandgap energy thereof is similar to that of Al_(0.13)Ga_(0.87)N to maintain an electron blocking function while the net polarization mismatch with respect to an adjacent quantum barrier layer is reduced. That is, conditions of a usable electron blocking layer according to the current embodiment include bandgap energy higher than the adjacent quantum barrier layer, and the net polarization smaller than or equal to the adjacent quantum barrier layer.

Table 1 below shows calculation results of the net polarization and the bandgap energy of the embodiments 1 and 2, as well as those of a related art Al_(0.13)Ga_(0.87)N electron blocking layer and a GaN quantum barrier layer. In this case, the electron blocking layer (Al_(0.3)In_(0.13)Ga_(0.57)N) of the embodiment 1 has the same bandgap energy as Al_(0.13)Ga_(0.87)N and the same net polarization as the GaN quantum barrier layer. Also, the electron blocking layer (Al_(0.25)In_(0.08)Ga_(0.67)N) of the embodiment 2 has the same bandgap energy as Al_(0.13)Ga_(0.87)N and the net polarization mismatch with the GaN quantum barrier layer, which is half the net polarization mismatch between Al_(0.13)Ga_(0.87)N and the GaN quantum barrier layer. In Table 1 below, QB represents a quantum barrier layer.

TABLE 1 Net Bandgap energy polarization Composition (eV) (C/m2) Embodiment 1 Al_(0.3)In_(0.13)Ga_(0.57)N 3.6588 −0.0339 Embodiment 2 Al_(0.25)In_(0.08)Ga_(0.67)N 3.6588 −0.0381 Comparison Al_(0.13)Ga_(0.87)N 3.6588 −0.0423 example 1 QB GaN 3.4200 −0.0339

FIG. 4 is a graph showing a simulation result concerning current changes according to a driving-voltage change in the embodiments 1 and 2 and the comparison example (Al_(0.13)Ga_(0.87)N electron blocking layer) selected as shown in Table 1. In this case, the emission wavelength is 450 nm and the temperature conditions is set to 300 K. However, an influence of a crystalline feature or the like of a semiconductor layer is not considered. Referring to FIG. 4, driving voltages of the embodiments 1 and 2 are lower than a driving voltage of the comparison example. Particularly, it can be seen that the embodiment 1 in which the electron blocking layer has the same net polarization as the quantum barrier layer shows the best result. In the case of the comparison result, the absolute value of the net polarization of the electron blocking layer is greater than that of an adjacent quantum barrier layer. Thus the magnitude of the positive of the electron blocking layer is greater than that of the quantum barrier layer at an interface between the electron blocking layer and the quantum barrier layer. Accordingly, the interface is positively charged, and this induces electrons flowing toward the electron blocking layer, degrading an electron blocking effect. In the embodiments 1 and 2, the net polarization mismatch does not exist or is reduced as compared to the comparison example at an interface between the electron blocking layer and the quantum barrier layer, thereby reducing the electron leakage over the electron blocking layer.

FIG. 5 is a graph of a simulation result of changes in electron leakage current according to a current change in the embodiment 1 (solid line) and the comparison example (dotted line). FIG. 6 is a graph of a simulation result of changes in internal quantum efficiency according to a current change in the embodiment 1 (solid line) and the comparison example (dotted line). Referring to FIG. 5, the amount of electron leakage currents is much smaller in the embodiment 1 than in the comparison example. This means that the electron blocking function is affected by the form of the energy level associated with the net polarization at the electron blocking layer. That is, according to the current embodiment, if the energy-level bending of the electron blocking layer is minimized, the electron blocking function can be improved. Referring to FIG. 6, as for the internal quantum efficiency at high currents, it can be seen that the quantum efficiency of the comparison example decreases by about 25% at 350 mA as compared to the maximum quantum efficiency, and the quantum efficiency of the embodiment 1 decreases by about 22%. Particularly, as compared to the comparison result, the internal quantum efficiency of the embodiment 1 is much higher than that of the comparison example, and efficiency improvement of about 49.5% is achieved at 350 mA.

A method for controlling Al and In contents is described according to the current embodiment. However, this method is merely one way of reducing the net polarization mismatch between the quantum barrier layer and the electron blocking layer or of reducing the degree of the energy-level bending of the electron blocking layer. Also, the method for reducing the net polarization mismatch may be also applied between the electron blocking layer and a p-type semiconductor layer, not just between the quantum barrier layer and the electron blocking layer. Also, the method for reducing the net polarization mismatch may also be applied to an interface of every layer adjacent to the electron blocking layer, e.g., between the electron blocking layer and a nitride spacer layer interposed between an active layer and the electron blocking layer.

FIG. 7 is a cross-sectional view of a modification version of the nitride semiconductor light emitting device of FIG. 2. A nitride semiconductor light emitting device 300 according to a current embodiment includes a substrate 301, an n-type nitride semiconductor layer 302, an active layer 303, an electron blocking layer 304 and a p-type nitride semiconductor layer 305. N-type and P-type electrodes 306 a and 306 b are respectively disposed on the n-type and p-type nitride semiconductor layers 302 and 305. Unlike the previous embodiment, the electron blocking layer 304 has a superlattice structure in which first and second layers 304 a and 304 b are alternately stacked. In this case, the first layer 304 a is formed of Al_(0.3)In_(0.13)Ga_(0.57)N of the embodiment 1, and the second layer 304 b may be formed of GaN of the comparison example.

According to the present invention, a nitride semiconductor light emitting device that can realize high efficiency by minimizing a net polarization mismatch between a quantum barrier layer and an electron blocking layer can be provided. Also, a high-efficiency nitride semiconductor light emitting device can also be provided by reducing the degree of the energy-level bending of the electron blocking layer.

While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims. 

1. A nitride semiconductor light emitting device comprising: n-type and p-type nitride semiconductor layers; an active layer disposed between the n-type and p-type nitride semiconductor layers and having a structure in which a plurality of quantum barrier layers and one or more quantum well layers are alternately stacked; and an electron blocking layer disposed between the active layer and the p-type nitride semiconductor layer, wherein the electron blocking layer has greater bandgap energy than a quantum barrier layer adjacent to the electron blocking layer among the plurality of quantum barrier layers, and has a net polarization equal to or smaller than that of the quantum barrier layer adjacent thereto.
 2. The nitride semiconductor light emitting device of claim 1, wherein the electron blocking layer has a net polarization which is smaller than or equal to that of GaN and is greater than that of Al_(0.25)Ga_(0.75)N.
 3. The nitride semiconductor light emitting device of claim 2, wherein the electron blocking layer has bandgap energy having the same magnitude as that of Al_(0.25)Ga_(0.75)N.
 4. The nitride semiconductor light emitting device of claim 1, wherein the electron blocking layer has a net polarization which is equal to that of the quantum barrier layer adjacent to the electron blocking layer among the plurality of quantum barrier layers.
 5. The nitride semiconductor light emitting device of claim 1, wherein a net polarization mismatch at an interface between the electron blocking layer and the quantum barrier layer adjacent to the electron blocking layer is smaller than a net polarization mismatch between GaN and Al_(x)Ga_((1−x))N (0.1≦x≦0.25).
 6. The nitride semiconductor light emitting device of claim 5, wherein the net polarization mismatch at the interface between the electron blocking layer and the quantum barrier layer adjacent to the electron blocking layer is half the net polarization mismatch between GaN and Al_(x)Ga_((1−x))N (0.1≦x≦0.25).
 7. The nitride semiconductor light emitting device of claim 1, further comprising a substrate for growth contacting the n-type nitride semiconductor layer, wherein the n-type nitride semiconductor layer is formed on a polar surface of the substrate.
 8. The nitride semiconductor light emitting device of claim 7, wherein the n-type nitride semiconductor layer is formed on a C (0001) plane of a sapphire substrate.
 9. A nitride semiconductor light emitting device comprising: n-type and p-type nitride semiconductor layers; an active layer disposed between the n-type and p-type nitride semiconductor layers and having a structure in which a plurality of quantum barrier layers and one or more quantum well layers are alternately stacked; and an electron blocking layer disposed between the active layer and the p-type nitride semiconductor layer, wherein the electron blocking layer has a constant energy level of a conduction band. 